1. Field of the Invention
The invention is generally related to offshore vessels and more particularly to the connection and disconnection of large floating objects in the offshore environment during higher sea states.
2. General Background
Different types of operations in the offshore environment present the need for the connection and disconnection of large floating objects. However, accomplishing such operations presents unique and extreme engineering and operational needs.
Two floating objects in unprotected water will have significant relative motion in six degrees of freedom in the higher sea states. Means for connecting two or more floating objects can be designed to restrict relative motion of the two objects in one, or more, of the six degrees of freedom. The rotational degrees of freedom are yaw, roll, and pitch. Resisting relative yaw of the two objects produces bending in the horizontal plane, resisting relative roll produces torsion, and resisting relative pitch produces hogging and sagging. The moment produced by resisting each rotational degree of freedom must be developed by a couple produced by a pair of connectors. A couple produces its greatest resisting moment when its moment arm is greatest. Therefore, the connectors producing each couple should be spaced as far apart as possible. The translational degrees of freedom are sway, surge, and heave. Resisting relative sway of the two objects produces transverse loads on the connectors, resisting relative surge produces longitudinal load on the connectors, and resisting relative heave produces vertical load on the connectors.
The couple forces required to resist the rotational degrees of freedom are much greater than the forces required to resist the translational degrees of freedom. For large objects, the magnitude of the couple required to resist relative pitch is so great that the marine connector must be designed to release pitch. Connectors designed to resist relative roll and yaw must be placed as far outboard to port and starboard as possible, but also must be designed to release pitch. The roll and yaw connectors may also be used to resist the relative translational degrees of freedom.
FIG. 1 shows two floating objects 10 rigidly connected at the four corners, as indicated by numeral 12. Rigidly connected in this usage means that the connection is not compliant. Although the connectors are ideally located at the extremities, the couple required to prevent relative pitch will be too great for practical design.
The impinging sea state applies most of the loadings to the connected objects. Therefore, the loads applied to the connected objects and the loads induced in the connectors are primarily dynamic. In some applications, the dynamic response of the connected objects can be a problem. For instance, where several objects 10 are rigidly connected bow to stern, as shown in FIG. 3, torsion has a ratio between its first and second modes of about two. Thus, if the first torsional mode were twenty seconds, the second torsional mode would be about ten seconds. Bending in the horizontal plane has a slightly better ratio of about two point seven seconds. Thus, if the first horizontal plane bending mode were twenty-seven seconds, the second mode would be ten seconds. These ratios are too low to avoid resonance with waves in the high energy spectrum. To adequately straddle the periods of the high energy spectrum waves, a ratio between the first and second modes of about six is required. For instance, if a structure were contrived with a twenty-seven second first mode in torsion, its second torsional mode would be about four point five seconds. At this ratio, the first and second torsional modes fall above and below, respectively, the periods of the high energy spectrum waves.
If the objects are rigidly connected in the example given above, then the connectors and the structure supporting the connectors must be designed for the dynamically amplified loadings induced by the torsional and horizontal plane bending modes. The higher loadings will also make the fatigue problems worse. An optional design would be to substitute compliant connectors for the rigid connectors, thereby altering the dynamic response of the connected units favorably. A major consideration in this option is that the design load for the connector is equal to the maximum capacity of the compliant element, provided the compliant element is designed so that it never reaches the end of its stroke.
Another problem is that the two floating objects to be connected must be brought into close enough alignment for the connectors to engage. The alignment operation is called docking and must be facilitated with a docking system. If the relative motions for which the docking and connection systems are designed are exceeded then the operation will have to wait for the lower motions that will come when the seas moderate. The connectors and the structure supporting the connectors must be designed to resist the forces that are induced by the impinging sea state. If the connected objects encounter a large storm that continues to worsen, or some other emergency occurs, the objects may have to be disconnected while the connectors are resisting large loads. Therefore, the connectors must be designed with the capability to disconnect under load. Once disconnected, the floating objects will quickly develop the relative motions of two independently floating objects. Therefore, the connection and docking systems must facilitate quick separation of the two objects to prevent impact between features on the two objects.
Also, where more than one connector is used between the floating objects, the connectors must be synchronized so they all connect or disconnect simultaneously. Otherwise, damage will occur.
An example follows of the loads that are encountered when connecting floating objects. For five floating objects, each being one thousand feet long and five hundred feet wide, the magnitude of the design load for rigidly mounted connectors, port and starboard, varies from twenty thousand metric tons to about one hundred thousand metric tons. The magnitude of the design load for compliant connectors, port and starboard, ranges from five thousand metric tons to ten thousand metric tons. The connectors must be capable of releasing while these types of loads are active. The inventors are not aware of connectors that meet these requirements.